Despite Goku's and Vegeta's outrageous leaps in strength in Dragon Ball Super, they are still spiky-haired larvae compared to the absolute power of the child king who rules over all of creation. Grand Zeno, also known as the Omni-King, is the single most powerful being in all of Dragon Ball.
His name, Zeno, is similar to the Japanese word for "almighty," an appropriate name indeed as he has displayed the ability to kill immortal beings and survive the destruction of an entire universe without harm. The Omni-King remains largely shrouded in mystery, but here is everything that's been revealed about him thus far.
It is not known where Zeno came from or how he became the Omni-King. The closest thing to an explanation regarding his past is the Grand Priest's declaration that the Tournament of Power would begin on the 3,135,500,603rd day of Zeno's rule, which equates to approximately 8.5 million years. Assuming the Dragon Ball multiverse is older than that, it would mean there was a time in which Zeno was not the Omni-King, potentially implying he appeared one day and seized power.
This would provide additional context to his role now, as Zeno does not appear involved in the creation of life, nor does he even seem to care about life in the first place. After all, there used to be 18 universes that made up the multiverse -- until Zeno destroyed six of them because he was in a foul mood. However, it has also been suggested he really did so to reduce the size of the multiverse and make it easier to manage. He was considering doing this again by eliminating another eight of the weaker universes until he had the idea to hold a tournament instead, which would allow one winning universe to survive.
Zeno's childlike behavior makes him difficult to understand. The Supreme Kai and Gods of Destruction quake with fear in his presence, and even the Angels treat him with formal respect. Meanwhile, Goku approaches him without bowing, tosses him around, and calls him Zen-chan or Zenny, unknowingly putting the entire cosmos in danger if Zeno takes the slightest offense to Goku's behavior. Ironically, this makes him take an immediate liking to the clueless Saiyan.
Ruling the multiverse with an iron fist appears to be a lonely job, as Zeno has expressed dissatisfaction with the reverence people show him. He even threatens Shin when the Supreme Kai attempts to speak on Goku's behalf in order to avoid possibly offending Zeno. True to his childlike nature, Zeno wants to have a friend, not a subordinate like Beerus who slams his own head through a table while bowing down to him.
Other than Goku and the Future Zeno brought back from Trunks' timeline as a playmate, only the Grand Priest seems able to act without fear in the presence of the Omni-King. Zeno appears very fond of him, heeding his advice and rewarding him with candy for a job well done. Zeno has even threatened his own guards for trying to protect him from Goku's approach, so his bond with the Grand Priest -- one of the few beings he has never intimidated into submission -- appears genuine.
Despite his behavior, Zeno appears to take his position in Dragon Ball Super seriously and will replace Gods of Destruction if he feels they are not properly performing their duties. He even foresaw the Tournament of Power's outcome, knowing the winning warrior would be virtuous enough to show compassion for the universes Zeno had destroyed. However, Zeno is also a realist, in that he was prepared to end all of reality if mortals had proven him wrong and made a selfish wish on the Super Dragon Balls. It's a good thing Android 17 changed his mind about that boat.
